1. Field of the Invention
This invention pertains generally to the field of fluid dynamic bearings, and more particularly to etching grooves in a hub used in a spindle motor of a disc drive to form such bearings.
2. Description of Related Art
Disc drives, including magnetic disc drives, optical disc drives and magneto-optical disc drives, are widely used for storing information. A typical disc drive has one or more discs or platters which are affixed to a spindle and rotated at high speed past a read/write head suspended above the discs on an actuator arm. The spindle is turned by a spindle drive motor. The motor generally includes a shaft having a thrust plate on one end, and a rotating hub having a sleeve and a recess into which the shaft with the thrust plate is inserted. Magnets on the hub interact with a stator to cause rotation of the hub relative to the shaft.
In the past, conventional spindle motors frequently used conventional ball bearings between the hub and the shaft and the thrust plate. However, over the years the demand for increased storage capacity and smaller disc drives has led to the read/write head being placed increasingly close to the disc. Currently, read/write heads are often suspended no more than a few millionths of an inch above the disc. This proximity requires that the disc rotate substantially in a single plane. Even a slight wobble or run-out in disc rotation can cause the disc to strike the read/write head, damaging the disc drive and resulting in loss of data. Because this rotational accuracy cannot be achieved using ball bearings, the latest generation of disc drives utilize a spindle motor having fluid dynamic bearings on the shaft and the thrustplate.
In a fluid dynamic bearing, a lubricating fluid such as gas or a liquid or air provides a bearing surface between a fixed member and a rotating member of the disc drive. Dynamic pressure-generating groove formed on a surface of the fixed member or the rotating member generates a localized area of high pressure or a dynamic cushion that enables the spindle to rotate with a high degree of accuracy. Typical lubricants include oil and ferromagnetic fluids. Fluid dynamic bearings spread the bearing interface over a large continuous surface area in comparison with a ball bearing assembly, which comprises a series of point interfaces. This is desirable because the increased bearing surface reduces wobble or run-out between the rotating and fixed members. Further, improved shock resistance and ruggedness is achieved with a fluid dynamic bearing. Also, the use of fluid in the interface area imparts damping effects to the bearing which helps to reduce non-repeat runout.
One generally known method for producing the dynamic pressure-generating grooves is described in U.S. Pat. No. 5,758,421, to Asada, (ASADA), hereby incorporated by reference. ASADA teaches a method of forming grooves by pressing and rolling a ball over the surface of a workpiece to form a groove therein. The diameter of the ball is typically about 1 mm, and it is made of a material such as carbide which is harder than that of the workpiece. This approach and the resulting fluid dynamic bearing, while a tremendous improvement over spindle motors using a ball bearing, is not completely satisfactory. One problem with the above method is the displacement of material in the workpiece, resulting in ridges or spikes, along the edges of the grooves. Removing these ridges, for example by polishing or deburring, is often a time consuming and therefore a costly process. Moreover, to avoid lowering yields great care must be taken not to damage the surface of the workpiece.
A further problem with the above method is due to a recent trend in disc drives toward higher rotational speeds to reduce access time, that is the time it takes to read or write data to a particular point on the disc. Disc drives now commonly rotate at speeds in excess of 7,000 revolutions per minute. These higher speeds require the shaft and the hub to be made of harder material. Whereas, in the past one or more of the shaft, the sleeve or the hub could be made of a softer material, for example brass or aluminum, now all of the components must frequently be made out of a harder metal such as, for example, steel, stainless steel or an alloy thereof. These metals are as hard or harder than the material of the ball. Thus, the above method simply will not work to manufacture fluid dynamic bearings for the latest generation of disc drives.
Another method for producing the grooves of a fluid dynamic bearing is described in U.S. Pat. No. 5,878,495, to Martens et al. (MARTENS), hereby incorporated by reference. MARTENS teach a method of forming dynamic pressure-generating grooves using an apparatus, such as a lathe, having a metal-removing tool and a fixture that moves the workpiece incrementally in the direction in which a pattern of grooves is to be formed. The metal-removing tool forms the grooves by carrying out a short chiseling movement each time the workpiece is moved. This approach, while an improvement over the earlier one in that it does not produce ridges that must be removed, is also not completely satisfactory. For one thing, this approach like that taught by ASADA is typically not suitable for use with harder metals, which in addition to being more difficult to machine are often brittle and can be damaged by the chiseling action. Moreover, because each groove or portion of a groove must be individually formed and the workpiece then moved, the process tends to be very time consuming and therefore costly. Furthermore, the equipment necessary for this approach is itself expensive and the metal-removing tool is subject to wear and requires frequent replacement.
A final method for producing the grooves involves a conventional etching process as described in U.S. Pat. No. 5,914,832, to Teshima (TESHIMA), hereby incorporated by reference. TESHIMA teaches a process in which the workpiece is covered with a patterned etch resistant coating prior to etching so that only the exposed portions of the workpiece are etched. While this approach avoids many of the problems of the previously described methods, namely the formation of ridges around the grooves and the inability to form grooves in hard metal, it creates other problems and therefore is also not wholly satisfactory. One problem is the time consumed in applying and patterning the etch resistant coat. This is particularly a problem where, as in TESHIMA, the resist coat must be baked prior to patterning or etching. Another problem is that the coating must be removed after etching. This is frequently a difficult task, and one that if not done correctly can leave resist material on the workpiece surface resulting in the failure of the bearing and destruction of the disc drive. Yet another problem with this approach is that each of the steps of the process requires the extensive use of environmentally hazardous and often toxic chemicals including photoresists, developers, solvents and strong acids.
Accordingly, there is a need for an apparatus and method for forming grooves in a workpiece made of a hard metal to manufacture fluid dynamic bearings suitable for use in a disc drive. It is desirable that the apparatus and method that allows the grooves to formed quickly and cheaply. It is also desirable that the apparatus and method not require expensive equipment or the use of a metal-removing tool that must be frequently replaced. It is further desirable that the apparatus and method not use an etch resistant material during manufacture that could contaminate the workpiece leading to the failure of the bearing and destruction of the disc drive.
The present invention provides a solution to these and other problems, and offers other advantages over the prior art.
The present invention relates to a method and apparatus for electrochemically etching grooves in an inner surface of a hub to form a fluid dynamic bearing that solves these problems.
In accordance with one embodiment, a cathode is provided having an electrically conductive substrate with an outer surface that corresponds to the inner surface of the hub, the outer surface including raised lands that correspond to the grooves to be formed in the inner surface of the hub. The substrate can be made of aluminum, brass, chromium, copper, nickel, steel, stainless steel, tin, zinc or alloys thereof. Preferably, the substrate is stainless steel. More preferably, the stainless steel is T-303 stainless steel or T-316 stainless steel. A layer of electrically insulating material covers the outer surface of the substrate between the raised lands to substantially preclude etching of the inner surface of the hub in areas corresponding to areas between the raised lands. Preferably, the layer of electrically insulating material includes an organic polymer bonded to the outer surface of the substrate. More preferably, the organic polymer comprises an adhesive, such as an epoxy resin. In one version of this embodiment, the inner surface of the hub comprises a cylindrical sleeve extending through the hub and a counterbored thrustplate cavity concentric with the sleeve. Preferably, the raised lands are arranged on the outer surface of the substrate to simultaneously etch grooves in the sleeve and the thrustplate cavity. Preferably, the raised lands are arranged on the outer surface of the substrate to form a fluid dynamic thrust bearing in the thrustplate cavity and at least one fluid dynamic journal bearing in the sleeve. The raised lands can be shaped and arranged on the outer surface of the substrate to etch a herringbone, an arcuate or a sinusoidal pattern of grooves in the inner surface of the hub.
In another aspect, the present invention is directed to a process of electrochemically etching grooves in an inner surface of an electrically conductive hub. The inner surface generally includes a cylindrical sleeve extending through the hub and a counterbored thrustplate cavity concentric with the sleeve. In the process, the hub and a cathode are held with a fixture adapted to hold the cathode within the hub so that there is substantially no contact between the cathode and the inner surface of the hub. An electrolyte is then allowed to flow between the cathode and the hub. The electrolyte can include one or more of water, a dilute acid, NaNO3 or mixtures thereof. Next, the cathode and the hub are coupled to an electrical current supply so that an electrical current is passed between the cathode and the hub to remove material from the inner surface of the hub. This can be accomplished, for example, by coupling the cathode to a negative terminal of the electrical current supply and coupling the hub to the positive terminal. Preferably, at least 10 A/cm2 is passed between the raised lands and areas of the thrustplate juxtaposed thereto. More preferably, less than 0.1 A/cm2 is passed between the electrically insulating layer and areas of the thrustplate juxtaposed thereto. In another preferred embodiment, grooves are etched simultaneously in the sleeve and in the thrustplate cavity. More preferably, the grooves are etched to simultaneously form a fluid dynamic thrust bearing in the thrustplate cavity and at least one fluid dynamic journal bearing in the sleeve.
In yet another aspect, the present invention is directed to an apparatus for electrochemically etching grooves in an inner surface of an electrically conductive hub. The inner surface generally has a cylindrical sleeve extending through the hub and a counterbored thrustplate cavity concentric with the sleeve. The apparatus includes a cathode having an electrically conductive substrate with an outer surface that corresponds to the inner surface of the hub. The outer surface includes a number of raised lands and a layer of electrically insulating material between the raised lands. A fixture holds the cathode with the hub so that there is substantially no contact between the cathode and the inner surface of the hub. Generally, the fixture comprises an electrically insulating body. A sealed electrolyte flowpath is provided to flow an electrolyte between the cathode and the hub. The flow-path can be adapted to flow a sufficient volume of electrolyte to remove material etched from the hub. An electrical current supply passes electrical current between the cathode and the hub so that material is removed from the inner surface of the hub in areas corresponding to the raised lands. In one preferred embodiment, the raised lands are arranged on the outer surface of the substrate to simultaneously etch grooves in the sleeve and in the thrustplate cavity. More preferably, the raised lands are arranged to form a fluid dynamic thrust bearing in the thrustplate cavity and at least one fluid dynamic journal bearing in the sleeve.
In still another aspect, the present invention is directed to an apparatus having a means for electrochemically etching grooves in the hub by passing electrical current through the electrolyte and the hub so that material is removed from the inner surface of the hub. In one version, the means for electrochemically etching grooves includes an electrically conductive substrate having an outer surface that corresponds to the inner surface of the hub. Preferably, the outer surface includes conducting means for passing electrical current from the substrate through the electrolyte to the hub, the conducting means juxtaposed to areas of the inner surface of the hub in which the grooves are to be formed. More preferably, the outer surface has an electrically insulating means for substantially preventing electrical current from passing from the substrate through the electrolyte to the inner surface of the hub in areas other than those in which the grooves are to be formed.
These and various other features as well as advantages which characterize the present invention will be apparent upon reading of the following detailed description and review of the associated drawings.